Postgresql - how to run a query on multiple tables with same schema - sql

Question

I have a postgres database that has several tables (a few hundreds). A subset Foo of the tables in the database have the same schema.
Ideally, I would like to create a stored procedure which can run a query against a single table, or against all tables in subset Foo.
Pseudocode:
CREATE TABLE tbl_a (id INTEGER, name VARCHAR(32), weight double, age INTEGER);
CREATE TABLE tbl_b (id INTEGER, name VARCHAR(32), weight double, age INTEGER);
CREATE TABLE tbl_c (id INTEGER, name VARCHAR(32), weight double, age INTEGER);
CREATE TABLE tbl_d (id INTEGER, name VARCHAR(32), weight double, age INTEGER);
CREATE TYPE person_info AS (id INTEGER, name VARCHAR(32), weight double, age INTEGER);
CREATE FUNCTION generic_func(ARRAY one_or_more_table_names)
RETURNS person_info
-- Run query on table or all specified tables
AS $$ $$
LANGUAGE SQL;
How could I implement this requirement in Postgresql 9.x ?

Answer 1

You should have a look at table inheritance in PostgreSQL, they allow exactly what you speak about.
For example, you could create a table parent_tbl:
CREATE TABLE parent_tbl (id INTEGER, name VARCHAR(32), weight numeric, age INTEGER);
Then link your tables to this parent table:
ALTER TABLE tbl_a INHERIT parent_tbl;
ALTER TABLE tbl_b INHERIT parent_tbl;
ALTER TABLE tbl_c INHERIT parent_tbl;
ALTER TABLE tbl_d INHERIT parent_tbl;
Then a SELECT query over parent_tbl will query all of tbl_x tables, while a query on tbl_x will query only this particular table.
INSERT INTO tbl_a VALUES (1, 'coucou', 42, 42);
SELECT * FROM tbl_a;
id | name | weight | age
----+--------+--------+-----
1 | coucou | 42 | 42
(1 row)
SELECT * FROM parent_tbl;
id | name | weight | age
----+--------+--------+-----
1 | coucou | 42 | 42
(1 row)
SELECT * FROM tbl_b;
id | name | weight | age
----+--------+--------+-----
(0 rows)
It is also possible to filter data from given children tables. For example, if you are interested in data coming from tables tbl_a and tbl_b, you can do
select id, name, weight, age
from parent_tbl
left join pg_class on oid = parent_tbl.tableoid
where relname in ('tbl_a', 'tbl_b');
EDIT : I put numeric for weight instead of double as this type is not supported on my server.

Answer 2

To create select query dynamically using items(table name) in an array you can use following select statement
SELECT string_agg(q, ' union all ')
FROM (
SELECT 'select * from ' || unnest(array ['tble_a','tble_b']) AS q
) t
Result:
string_agg
---------------------------------------------------
select * from tble_a union all select * from tble_b
You can create the function that returns table with columns
id INTEGER
,name VARCHAR(32)
,weight numeric
,age INTEGER
P.S: I am avoiding TYPE person_info
function:
CREATE
OR REPLACE FUNCTION generic_func (tbl varchar [])
RETURNS TABLE ( -- To store the output
id INTEGER
,name VARCHAR(32)
,weight numeric
,age INTEGER
) AS $BODY$
DECLARE qry text;
BEGIN
SELECT string_agg(q, ' union all ') --To create select query dynamically
INTO qry
FROM (
SELECT 'select * from ' || unnest(tbl) AS q
) t;
RAISE NOTICE 'qry %',qry; --optional
RETURN query --Executes the query to the defined table
EXECUTE qry;
END;$BODY$
LANGUAGE plpgsql VOLATILE
Usage:
select * from generic_func(array['tbl_a','tbl_b','tbl_c','tbl_d'])
Result:
id name weight age
-- ---- ------ ---
2 ABC 11 112
2 CBC 11 112
2 BBC 11 112
2 DBC 11 112
and
select * from generic_func(array['tbl_a'])
Result:
id name weight age
-- ---- ------ ---
2 ABC 11 112